In X-ray equipment, various means are used to bombard electrons onto a positively charged surface, referred to as an anode or an X-ray target, and thereby generate the X-rays. There are both stationary and rotating targets available commercially. The focal track is the portion of the surface of the target that is bombarded by the electrons.
Tungsten alone or tungsten alloyed with other metals are commonly used in X-ray targets. Metals which are sometimes alloyed with the tungsten are small amounts for example of rhenium, osmium, irridium, platinum, technetium, ruthenium, rhodium and palladium. X-ray targets formed wholly from tungsten alone, or tungsten alloys where tungsten is the predominant metal are undesirable because of the high density and weight of the tungsten. In addition, tungsten is notch sensitive and extremely brittle and is thereby subject to catastrophic failure with resultant damage to the usually delicate equipment with which the target is used, and possible injury to the patient or personnel using the equipment.
Because of the shortcoming of targets made wholly of tungsten alloys which contain relatively expensive alloying elements, attempts have been made to use tungsten or tungsten alloys only for the focal track layer of the target and to support this track on a substrate that is compatible with tungsten and at the same time is less susceptible to cracking, is of a lower density and if possible less costly. For the material to be compatible it must not melt or rapidly alloy with tungsten at the sintering temperature, it should match the coefficient of thermal expansion of tungsten as closely as possible, its pressing and sintering characteristics should also closely match those of the tungsten alloy powder and finally it must have good thermal conductivity. Unalloyed molybdenum meets all these requirements but it is not sufficiently strong at the elevated operating temperatures to always prevent warping and distortion of the tungsten focal track. If this distortion is severe enough a point will be reached at which the X-rays generated on the face of the focal track are no longer directed towards the X-ray emission window very specifically located in the wall of the X-ray tube. If this warpage continues, it eventually leads to an unacceptable drop-off in X-ray output. Molybdenum, however, is ductile and tough enough to nearly always resist extensions of cracks that inevitably form in the tungsten focal track layer due to the excessive thermal stresses imposed therein by the high energy electron bombardment. What is required, therefore, is a way of stiffening the molybdenum substrate without sacrificing its resistance to crack propagation and its other desirable properties.